1. Field of the Invention
This invention pertains to thin film composite materials which include a metal layer interposed between a transparent film or coating and another film or coating. More particularly the invention pertains to the above composite films which are de signed to allow light from a laser (e.g., solid state laser) to interact with a metal film contained therein after passage of the light through one of the films or coatings in contact with the metal layer.
2. Background Information
A variety of thin film composite structures are currently available which are designed to interact with laser energy from solid state lasers. Typically these composite structures include a metal layer sandwiched between two thin film layers, at least one of which is transparent to the laser light so that the laser light can readily pass therethrough and interact with the metal layer within the composite. Such composite films are well known and are sold to a variety of industries for different applications. Thus such composite film structures may include additional layers, materials and may have particular structural features depending upon the specific end use application. However, they all share the above-described composite as a common feature, i.e., they all contain the metal layer in contact on one side thereof with a layer which is transparent to the laser light and another layer which need not be transparent to the laser light on the other side thereof.
Composite films which share the above-described common feature are used as imaging media and may find utility as printing plates, photo masks, dye transfer films, etc. The images are formed by selectively ablating away portions of a semitransparent metal layer located within the composite through the use of light energy such as laser light. In these applications it is desirable to maximize light energy coupling to the metal layer to allow use of lower output and less costly lasers.
As noted above, light must first pass through a transparent layer before it can reach the metal and interact therewith. However, the metal, such as titanium which is frequently used as the ablated metal in such composite films, is capable of reflecting at least a portion of the incident light and thus undesirable reflection of light will occur at the transparent layer/metal interface. Such reflection at the transparent layer/metal interface is undesirable because it reduces the amount of available light needed for interaction with the metal. Thus, it is highly desirable to reduce the amount of light which is reflected at the metal/transparent layer interface.
Various types of antireflection coatings are available for antiflecting a metal layer (i.e., for reducing the reflection of light therefrom). However, it is extremely difficult to antireflect a metal layer which is imbedded in a composite matrix because the metal is not in direct contact with the air. In other words, the transparent layer through which the light passes before impinging on the surface of the metal, reduces the types of materials which can be used as an antireflecting layer on the metal. Thus, a simple antireflecting coating on the metal within the composite would be highly desirable particularly if the desired antireflecting. coating or layer is inexpensive and easy to apply on the metal. It would be, especially desirable to achieve the desired antireflection through the use of a single antireflection layer on the metal within the composite.
It is an objective of the present invention to reduce the amount of light reflected by metal at the metal/transparent interface of a thin film composite.
This and other objectives are achieved by applying an antireflection coating of material having an index of refraction greater than 2.5 and an extinction coefficient which is less than 0.5 at 600-1200 nm wavelengths. Preferably the antireflection layer is a coating of an inorganic compound selected from the group consisting of an oxide of copper, ferric oxide (Fe2O3) and silicon carbide (SiC) on the metal which is to be antireflected. Preferably the antireflective coating is a single layer. The preferred material for the antireflection coating is cuprous oxide.
The present invention is applicable to any of the known composite film structures described above which include a metal layer therein. Typically such composite film structures include a transparent layer such as a transparent polymer adhered to one surface of a metal layer and another layer such as a polymeric layer which need not be transparent, adhered to the other side of the metal layer. The invention is also applicable to any film assembly which contains a transparent film or coating adhered to a metal layer wherein the assembly is used as a subcomponent of a structure in which it is desired to antireflect light at the transparent film/metal interface. Various manufacturing methods are known for producing such polymer/metal/polymer thin film composite structures. For example, the metal may be coated by any known process (e.g., sputtering, thermal evaporation, electron beam evaporation, etc.) onto the surface of one film to produce a metal/film structure. Then the exposed metal surface of the metal/film structure may be coated to form a film thereon by any known method. Preferably each film adhered to the metal is a polymeric film such as the numerous polymeric films currently used for composite film structures.
In order to practice this invention it is only necessary to modify the known methods of making the aforementioned composite films by further including an antireflecting coating on the metal so that the light which enters the composite film first passes through the transparent layer and then passes through the antireflecting layer which is in contact with the metal. Interposing an antireflecting layer between the transparent layer and the metal layer reduces the amount of light which is reflected by the metal and thereby increases the amount of available light for interaction with the metal.
As noted above, although it is known to apply an antireflecting layer on a layer which is to be antireflected, such layers are placed on the top of the structure (i.e., on the surface thereof) so that light enters the structure by first passing through the antireflecting layer, not by first passing through a transparent film such as a transparent polymeric layer. The use of such prior art antireflecting layers within the composite structure, as opposed to the use of an antireflecting coating on the surface of the structure, will not produce the desired antireflection of the metal layer located within the composite. Without being bound to any particular theory, it is believed that the materials used in prior art antireflecting layers are inadequate to antireflect the metal located beneath a transparent film due to an inadequate index of refraction of such prior art antireflection materials.